Teach Testing/External to use ExternalData

Configure the ExternalData_SOURCE_ROOT to use the top of the VTK source
tree.  Add the VTKData target to prepare referenced data when building
the tests.  This completes the Testing/External changes started by
commit 43f0b283 (Add ExternalData infrastructure, 2013-05-06).

Change-Id: Icb16a5420ae895a1d85c0a9eb253bbc3ed2897b9
Merge branch 'use-ExternalData' into master

d0550e61 Teach Testing/External to use ExternalData

Change-Id: Ica1368328775edde648a23bde44e19d14d5a5730